Many techniques have been devised to reduce interference signals that otherwise may degrade communication systems, such as receivers or transmitters. For example, satellite signals received by user terminals are susceptible to interference contained in satellite downlinks as well as interference received from terrestrial sources.
A receiver may be configured to receive and process signals that have broad bandwidth spectra and powers within a certain, expected, range. For example, a receiver on a satellite may be configured to receive a group of signals that share a common region of the electromagnetic spectrum, and are multiplexed with one another using techniques known in the art. In the multiplexing technique known as code division multiple access (CDMA), each signal of the group is encoded with a unique code, and spread over the same selected portion of the spectrum as the other signals in the group. The receiver receives the group of signals, and then decodes one or more of the signals from others in the group using a priori knowledge about the unique code(s) of those signals. Alternatively, in the multiplexing technique known as frequency-division multiple access (FDMA), each signal of the group is assigned a different sub-portion of the region of the spectrum than the other signals in the group. The receiver receives and processes the group of signals, and then differentiates one or more of the signals from others in the group using a priori knowledge about the spectral sub-portion(s) of those signals. When interference protection is required, the FDMA signal collection is hopped over a wider frequency range. The hopping sequence is known to the user and the receiver, but not to an adversary. As a result, the object of frequency hopping is to force the adversary to dilute its resources over the wider bandwidth which reduces the adversary's effectiveness. Yet another multiple access technique is time division multiple access (TDMA) where individual users are assigned a time period during which that user has exclusive use of the entire bandwidth. Other users are likewise assigned their exclusive time periods that are selected so that time periods of the individual users do not overlap. The groups of signals received in CDMA, frequency hopped FDMA, and TDMA may be considered “broad bandwidth” signals because the groups of signals occupy a portion of the electromagnetic spectrum that is significantly broader than normally would be used for a single, non-multiplexed signal, that is, a “narrow bandwidth” signal.
In CDMA, frequency hopped FDMA, and TDMA, the overall power of the group of signals received by the receiver preferably is sufficiently higher than any noise sources that may be present to yield a sufficient signal-to-noise ratio (SNR) to communicate signals with adequate fidelity as measured by BER (Bit Error Rate) values. At the same time, the overall power of the group of signals also preferably is sufficiently low that the receiver may process the signals without distortion. Specifically, as is known in the art, receivers have a linear range of operation and a nonlinear range of operation. If a signal input to the receiver has a power that falls within the linear range of the receiver, then the receiver processes the received signal collection without distortion. However, if a signal input to the receiver has a power that falls within the nonlinear range of the receiver, then the received signal collection is distorted and communication performance is degraded.
Signals other than the desired group of signals that the receiver receives may be referred to as “interference,” may be intentional or unintentional, and may have a broad bandwidth or a narrow bandwidth. If the receiver receives interference that falls within the same portion of the electromagnetic spectrum as the desired group of signals, then the receiver may not distinguish the interference from the group of signals again degrading communication performance. However, if the power of the interference is sufficiently high that nonlinear receiver operation occurs, not only may the interference obscure desired spectral components but also cause additional signal distortion. This additional receiver distortion may include suppression of desired signals and generation of intermodulation products between design signal components and the interference, resulting in additional degradation in receiver performance.
A receiver may have features intended to reduce the effects of such interference. For example, the receiver may be designed so as to increase its linear range, and thus reduce the risk that interference may cause distortion, e.g., by providing circuitry that remains linear at higher input power levels. However, increasing the linear range of the receiver may be expensive, and also may require a larger power supply to operate the modified circuitry.
Another known approach for reducing the effect of narrow bandwidth interference on reception of a broad bandwidth desired signal uses adaptive notch filter techniques. Specifically, a notch filter may be applied to the received signal prior to amplification so as to block the region of the spectrum where the interference is located. The amplitude, width, and spectral location of the notch filter may be adaptively modified over time by varying weighting coefficients, which may be iteratively derived using a gradient process based on an optimization criterion, such as maximum signal to noise plus interference ratio (SNIR). Such adaptive notch filter techniques have been widely applied. However, its iterative nature makes this approach is relatively slow, and thus less able to respond to rapidly changing interference.
Many of today's systems use digital implementations in processing signals. These digital implementations utilize an A/D (Analog to Digital converter) to process analog signals into digital data streams for subsequent processing. Such A/D converters become nonlinear for power inputs that are much lower than would cause nonlinearities in otherwise similar analog circuitry. Analog AGC (Automatic Gain Control) circuitry inserted prior to the A/D can be used to maintain the linearity of digital circuitry. Such AGC circuitry uses time averaging techniques to reduce the analog levels at the A/D input, and like conventional adaptive notch filters, the time averaging limits the ability to respond to dynamically varying interference signals.
Transmitters also can be negatively impacted by interference that spectrally overlaps a desired signal. Many systems use a transponder architecture where received signals are processed and routed to a transmitter for rebroadcast. Such systems may receive and transmit one or more signals and can be degraded by one or more interfering sources. The transmitter power levels, however, can be controlled to maintain linear system operation. Interfering signals in addition to the desired signals can result in nonlinear transmitter operation which distorts and degrades the desired transmitted signals.
The CDMA signal format is an example of spread spectrum modulation wherein user signals are spread over a much wider bandwidth than needed to convey the information in the user's signal. One advantage of spread spectrum modulation is protection from interference achieved by processing the user-unique codes. Similar interference protection may be achieved in FDMA formats by frequency hopping the user assigned frequency slots over a wide bandwidth in a pseudorandom sequence of frequency hop codes known to both the sender and receiver. Signal error correcting coding and interleaving techniques further add to the interference protection and are commonly used. These interference protection techniques are known in the art, but their benefits depend on linear receiver operation. The effectiveness of these techniques is significantly degraded by receiver nonlinearities.